The present invention relates in general to a method for acoustic well-logging and more particularly it relates to a method for determining subterranean formation permeability from acoustic tube wave energy.
It is well-known that measurements of the conductivity and porosity of a formation are important in determining whether hydrocarbons are located therein, the hydrocarbon being found as non-conductive fluids in porous rock formations. It is also well-known that hydrocarbons are generally not extractable from porous rock formations unless those formations are also permeable. Accordingly, after hydrocarbons are detected, it has been generally the practice to obtain a sidewall core of a given formation in order to measure permeability in the laboratory. Such a technique is time consuming and unduly expensive.
Techniques of acoustic well-logging are also well known and the possible applicability of such techniques to the determination of the permeability of a formation was predicted by Rosenbaum in the article "Synthetic Microseismograms: Logging In Porous Formations", Geophysics, Volume 39 No. 1 (February, 1974).
Rosenbaum investigated numerically, the ideal case of a borehole filled with an inviscid fluid surrounded by a formation that is porous and which conforms to Biot's theory as set forth in "Propagation of Elastic Waves In A Cylindrical Bore Containing A Fluid", Journal of the Acoustic Society of America, Volume 23, 1952, pages 997-1005. Rosenbaum's investigation showed that the effect of the pore-fluid mobility on the calculated response is large and can be measured with an appropriate logging tool. He further predicted that the relative amplitude of a tube wave obtained from an acoustic logging tool with a wide band frequency response would depend upon the permeability of the formation.
One such acoustic logging tool utilized in the determination of formation permeability is described in U.S. Pat. No. 4,432,077 to Alhilali and Zemanek, Jr. As the logging tool traverses a borehole, a low frequency acoustic source transmits acoustic energy through the formations surrounding the borehole. A spaced apart receiver measures such acoustic energy at a plurality of locations along the borehole. Changes in amplitude of acoustic tube waves are observed from one of a plurality of locations in the borehole with respect to another of such locations. A change in tube wave amplitude gives a measure of formation permeability. More particularly, a pair of spaced-apart receivers may be utilized wherein the ratio of the tube wave amplitude measured by one of the detectors to the tube wave amplitude measured by the other of the detectors is observed at each of a plurality of locations in the borehole. A change in such ratio also provides a measure of formation permeability.
Another such acoustic logging tool utilized in the determination of formation permeability is described in U.S. Pat. No. 4,575,828 to Williams. The amplitude and travel time relationships of tube waves detected by a pair of spaced apart receivers within the logging tool are recorded as the tool traverses at least along an identified location of a naturally fractured formation. Total permeability of the naturally fractured formation is estimated in accordance with a predetermined correlation between permeability and the recorded tube wave amplitude relationship. Matrix permeability of the naturally fractured formation is estimated in accordance with a predetermined correlation between permeability and the recorded tube wave travel time relationship. That portion of the estimated total formation permeability that is attributable to the naturally occurring fractures in the formation is identified by the difference between such estimated total formation permeability and said estimated matrix permeability. In a further aspect, the recording of the amplitude relationship between tube waves detected by the pair of spaced-apart receivers includes the recording of the ratio of the tube wave amplitudes. The recording of the travel time relationship between the pair of spaced-apart receivers includes the recording of the difference of the tube wave travel times. Total formation permeability is estimated from a predetermined correlation established between recordings of permeability taken from core samples adjacent a naturally fractured formation in a select borehole and the ratio of tube wave amplitudes measured across the naturally occurring fractures by a pair of spaced-apart receivers in such select borehole. Matrix permeability is estimated from a predetermined correlation established between recordings of permeability taken from core samples adjacent a naturally fractured formation in a select borehole and the difference of the tube wave travel times across the naturally occurring fractures by a pair of spaced-apart receivers in such select borehole.
While such acoustic logging tools have been utilized in the determination of formation permeability, a need still exists for improved techniques of processing the measured acoustic energy signals for more accurate determinations of such permeability.